Methylmalonic acidemia (MMA) is a metabolic disorder characterized by the abnormal buildup of the metabolic byproduct methylmalonic acid in patients. MMA causes developmental delay, intellectual disability, kidney disease, coma, or even death. MMA is also referred to as methylmalonic aciduria. It has an estimated incidence of 1 in 50,000 to 100,000. Current treatment for MMA is primarily via dietary control to limit the usage of metabolic pathways that lead to methylmalonic acid formation. In serious cases, kidney and liver transplants have also been performed to provide a new reservoir of cells that can properly metabolize and remove the methylmalonic acid. However, none of these treatments completely or reliably controls the disorder. As such there is a need for improved therapy to treat MMA.
The principal gene associated with MMA is methylmalonyl-CoA mutase (NM_000255; NP_000246; also referred to as MCM or MUT). MCM is a metabolic enzyme (E.C. 5.4.99.2) that plays a critical role in the catabolism of various amino acids, fatty acids, and cholesterol. MCM's biological function is to isomerize L-methylmalonyl-CoA into succinyl-CoA, a Krebs cycle intermediate. MCM localizes to the mitochondria of cells, exists as a homodimer in its native form and is adenosylcobalamin-dependent. The precursor form of human MCM is 750 amino acids, while its mature form is 718 amino acids—a 32 amino acid leader sequence is cleaved off by mitochondrial importation and processing machinery. This leader sequence is variously referred to as MCM's mitochondrial targeting peptide, mitochondrial targeting sequence, or mitochondrial transit peptide.
A complete or partial loss of MCM function leads to buildup of abnormal metabolites and metabolic intermediates upstream of MCM, such as methylmalonic acid, propionyl-carnitine, acetyl-carnitine, propionyl-CoA, D-methylmalonyl-CoA and L-methylmalonyl-CoA. For example, loss of MCM has been reported to lead to a 1000-fold increase in the methylmalonic acid. Nonetheless, there is no currently available therapeutic to treat MMA.